In the current mirror chapter of All About Circuits, in the last part it is said that to get twice load current increase area of the transistor. How is this possible. How does emitter current in a current mirror depend upon area of the transistor?
This discussion involves creating the circuit, i.e. building the transistors, in the silicon itself. This is the task of the integrated circuit (IC) designer. Most of us will never see transistors or circuits at that level, we buy them only after they are packaged. Therefore, we do not have the ability to change the physical size of the individual transistor, but can choose a different transistor to match the specification of our individual circuit.IC designers avoid some resistors by replacing load resistors with current sources. A circuit like an operational amplifier built from discrete components will have a few transistors and many resistors. An integrated circuit version will have many transistors and a few resistors. In Figure below One voltage reference, Q1, drives multiple current sources: Q2, Q3, and Q4. If Q2 and Q3 are equal area transistors the load currents Iload will be equal. If we need a 2·Iload, parallel Q2 and Q3. Better yet fabricate one transistor, say Q3 with twice the area of Q2. Current I3 will then be twice I2. In other words, load current scales with transistor area.
Yes, but in the current mirrors you were referring to, larger area gives more current at the same junction voltage.
Actually aren't current mirrors acting like constant current sources(for a constant junction voltage). As per I understood without changing the junction voltage, current won't increase.The larger current going through the transistor is due to a multiple loads (larger current draw) now being attached to one place instead of two as with two transistors. Changing the area allows the transistor to handle the larger load.
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by Jake Hertz